The production of high purity niobium (Nb) or tantalum (Ta) compounds still relies on processes that require highly acidic conditions and concentrated HF or NH 4 F solutions. Recent progress in the chemistry of group V elements, along with the evolution of the environmental concerns, call for the development of more sustainable industrial processes for these strategic elements. In sharp contrast with the historical fluoride approach chosen by the Nb and Ta refining industry, there has been a recent surge in the literature concerning the chemistry of these elements in diluted alkaline or oxalate media. We here propose an innovative liquid-liquid extraction process that is amenable to separate Nb and Ta from alkaline feed solutions, can be operated at low acidity and that does not require any fluoridebased chemical. The developed strategy is as follows: first, Nb and Ta are co-extracted from their alkaline solution (pH 12) using a quaternary ammonium salt (Aliquat® 336) dissolved in an organic diluent; then, Nb is selectively back-extracted in a mixture of diluted nitric and Page 2/31 oxalic acids; and finally, Ta is stripped in a diluted nitric media. The process also affords purifying Nb from Ti, Fe and Na. Following an optimization at laboratory scale, the full process has been validated during continuous running tests (160 h at 1.4 L.h-1) using industrial Nb-Ta caustic solutions coming from the development of the so-called Maboumine process. The developed process works at room temperature, low acidity ([H + ] < 1 M), exhibits high recovery yields, and allows producing Nb 2 O 5 •nH 2 O with purity higher than 99.5 %. The process also yields a tantalum product (Ta 2 O 5 •nH 2 O with >20 wt% Ta) even when starting from a diluted feed (39 ppm Ta). The proposed strategy paves the way for new Nb and Ta hydrometallurgical processes operating under mild chemical environments and represents a concrete alternative to the current fluoride-intensive industrial methods. Highlights A new process for separating Nb and Ta in fluoride-free media is proposed High-purity Nb oxide (99.5 %) can be produced Nb and Ta can be recovered and separated starting from an alkaline feed solution The process works at room temperature and under mild chemical conditions
Oxalate-based aqueous media represent one of the rare options for solubilizing macroscopic amount of niobium (Nb) and tantalum (Ta) without using toxic fluoride-based mixtures. Recent progress in the hydrometallurgy of Nb and Ta also highlighted the potential of oxalate-nitrate media for the separation of Nb and Ta. Nonetheless, the resulting purified aqueous solutions, containing Nb or Ta in HNO3-H2C2O4 mixtures, need to be further processed in order to yield Nb and Ta solid products that can be commercialized. Furthermore, oxalic acid is relatively expensive in the frame of the hydrometallurgy of Nb and its recycling is necessary. In this study, the precipitation of Nb and Ta, as well as their usual minor impurities (Ti and Fe), from oxalic-nitric acid solutions has been investigated for the first time. Neutralization of Nb-HNO3-H2C2O4 mixtures to pH 7-8 by concentrated NaOH or NH4OH was found to be effective at decomposing the oxalate complexes ([NbO(C2O4)3] 3and [NbO(C2O4)2(H2O)]-) and precipitating Nb in the form of Nb2O5•nH2O while leaving the Page 2/27 oxalate ions in solution. Taking advantage of the solubility difference between nitrate and oxalate salts, the subsequent concentration of the filtrates yields to the recovery of the oxalates as Na2C2O4 or (NH4)2C2O4•H2O and a concentrated solution of sodium or ammonium nitrate. The developed method exhibits high precipitation yields (>99.9%) for Nb and Ta and high recovery yields (>99%) for the nitrates and oxalates. The process was optimized at the laboratory scale (~50 mL) and then validated on industrial Nb and Ta solutions (~225 L) with successful production of purified Nb2O5•nH2O and Ta2O5•nH2O, recovery of the oxalates in the form of Na2C2O4 or (NH4)2C2O4•H2O and a concentration solution of nitrates. Taking together, the proposed precipitation method and the fluoride-free liquid-liquid Nb-Ta separation recently reported pave the way for more suitable hydrometallurgical processes for Nb and Ta. [19] K[NbO(C2O4)2(H2O)2]•3H2O A[NbO(C2O4)2(H2O)2]•2H2O (A = Rb, Cs) Mg[NbO(C2O4)2(H2O)2]2•8H2O
Separation processes based on room temperature ionic liquids (RTIL) and electrochemical refining are promising strategies for the recoveryo fl anthanides from primary ores and electronic waste. However,t hey require the speciation of dissolved elements to be knownw ith accuracy. In the present study,E uc oordination and Eu III /Eu II electrochemical behavior as af unction of water contenti n1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm] [NTf 2 ]) was investigatedu sing UV-visible spectrophotometry, time-resolvedl aser fluorescence spectroscopy,e lectrochemistry,a nd X-ray absorption spectroscopy. In situ measurements were performed in spectroelectrochemicalc ells. Undera nhydrous conditions, Eu III and Eu II were complexed by NTf 2 ,f orming EuÀOa nd EuÀ(N,O) bonds with the anion sulfoxide functiona nd Na toms, respectively.T his complexation resulted in agreater stabilityofEu II ,and in quasi-reversible oxidation-reduction with an E 0 'p otential of 0.18 V versust he ferrocenium/ferrocene (Fc + /Fc) couple. Upon increasingw ater content,p rogressivei ncorporation of water in the Eu III coordination sphereo ccurred. This led to reversible oxidation-reduction reactions, but also to ad ecreasei n stabilityo ft he + II oxidation state (E 0 ' = À0.45 Vv s. Fc + /Fc in RTIL containing 1300 mm water).
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